The control of unwanted immune responses is a critical issue in the treatment of diseases such as autoimmune diseases, transplant rejection, allergic diseases, and some cancers. The activity of overly aggressive T cells can be controlled by immunosuppression or by the induction of immunological tolerance. Tolerance is defined as a state where the immune system is made unresponsive to an antigen, whereas immunosuppression, which decreases the immune response to antigens, usually requires the continued use of medication. In organ transplantation, T cells play an essential role in the immune response to alloantigens. Current immunosuppressive regimes commonly involve the use of corticosteroid, cyclosporin or rapamycin, which block the transcription of IL-2, a key growth factor for T cells, or inhibit IL-2 dependent proliferation. However, a number of monoclonal antibodies which either act as T cell-depleting agents (e.g. CD3, CD4, CD8), or as inhibitors of the cytokine signaling or the co-stimulatory pathways of T cells (e.g. CD25, B7-1, B7-2, CD152, CTLA4) have demonstrated effectiveness in reducing the incidence of rejection with limited side effects or toxicity. Some of these agents have been shown to have some degree of success in treating autoimmune disease and in prolonging graft survival.
Apoptosis is widely believed to be of vital importance for maintaining the proper function of the immune system and a major mechanism to remove unwanted cells (Kabelitz et al. Immunol. Today 14:338-340 (1993); Raff, Nature:356:397-399 (1992)). Various signals originating from either inside or outside a cell influence the life and death of the cell. Antibodies against T cell surface molecules such as Fas (or CD95, MW=43 kD), TNFR2 (MW=75 kD), CD2 (MW=45 kD) and CTLA-4 (MW=33 kD) to induce the apoptosis of T cells (Osborne, Curr. Opin. Immunol. 8:245-248 (1996); Lin et al. J. Immunol. 158:598-603 (1997); Zhang et al. Nature:377:348-350 (1995); Lai et al. Eur. J. Immunol. 25:3243-3248 (1995); Mollereau et al. J. Immunol. 156:3184-3190 (1996); Gribben et al. Proc. Natl. Acad. Sci. USA 92:811-815 (1995)). Attempts to use Fas and TNFR2 molecules to control unwanted T cells have been hampered by the fact that these two molecules are expressed not only on immune cells, but also on several other important organ systems like liver. This expression pattern potentially limits the therapeutic application of these two antibodies (Ogasawara et al. Nature 364:806-809 (1993); Pfeffer et al. Cell:73:457-467 (1993); Engelmann et al. J. Biological Chemistry 265:14497-14504 (1990)).